The recent proliferation of solid-state lighting has created a need for high performance wavelength conversion materials. The standard approach is to form wavelength conversion materials (e.g. phosphors) using solid state processing as known in the art. These phosphors are then ground down to powders in the micron size range and deposited on a surface using a variety of deposition techniques such as settling, encapsulation, and spray coating. Large extended area devices such as cathode ray tubes (CRTs), fluorescent lighting, and plasma displays require a large amount of standard high volume phosphors. These phosphors can be acquired for less than $100/Kg for the large extended area devices. Though relatively inexpensive, the phosphors generated using this method suffer from high levels of dislocations and lattice defects. In addition, the compositional purity is also difficult to maintain. In the majority of cases, this does not represent a major problem because of the reduced excitation levels. It has been shown, however, in accelerated aging studies that very high excitation levels can degrade the output luminescence of powdered phosphors severely and impact overall life performance. These levels of high excitation exist within solid-state lighting applications. This is mainly due to the small size and concentrated flux density of the LED die itself. It is also true that in the case of individual or arrays of LEDs, phosphor usage is measured in milligrams rather than grams. It is apparent that higher performance wavelength conversion materials are needed and can be afforded for applications like solid-state lighting. The expense of higher performance materials can be absorbed and even offset if the problems associated with the degradation and reduced conversion efficiency of phosphors used in solid-state lighting can be overcome.
Several material characteristics contribute to this degradation and/or loss in efficiency problem such as lattice defects, out-gassing, and compositional purity. It has been shown that polycrystalline and mono-crystalline phosphor films either grown on a substrate or as single crystal boules tend to exhibit much better luminosity and life characteristics than powders. In addition, every phosphor has a thermal quenching level that can degrade the output at the temperatures created by elevated excitation levels. In the case of powdered phosphors, this can be a major issue because the phosphor particles are usually isolated from any reasonable thermal conduction path. At very high excitation levels the energy associated with less than unity quantum efficiency and Stokes shift losses can induce a significant localized thermal rise within the phosphor particles. The need exists for creation of an improved thermal conduction path for the luminescent material. Also, because the particles are roughly spherical the packing density can be significantly degraded. This affects the tradeoff between maximum absorption/conversion of the excitation energy and reabsorption of the emitted light. The scattering created by the use of a powder can reduce the overall light output due to the backscattering and subsequent absorption of the generated light.
Mueller-Mach et al. in U.S. Pat. No. 6,696,703 disclose the deposition of a thin film phosphor directly on the LED die. However, as-deposited thin film phosphors have relatively poor wavelength conversion efficiency. A high-temperature annealing step is required in order to properly activate the phosphor. This annealing step can damage the semiconductor layers of the LED. In addition, the absorption cross-sections of most thin film phosphors are low, especially for blue and near ultraviolet (UV) excitations typically used within solid-state lighting. It is neither economical nor practical in most cases to create a sufficiently thick layer of luminescent material directly on the LED. Another drawback to depositing a phosphor directly on the LED die is that a large portion of the light generated within a deposited phosphor layer can be trapped due to total internal reflectance. The need therefore exists for a method to utilize high performance phosphors within an LED package such that the best phosphor can be used efficiently (e.g. with sufficient quantity, minimal backscatter, and maximum light extraction). The need also exists for a method to fabricate high efficiency phosphors without damaging the LED semiconductor layers.
Another important aspect of phosphors relates to characterization and overall device efficiency. Phosphors are typically characterized in terms of quantum efficiency and Stokes shift losses. As an example, a powder phosphor layer is deposited on a glass surface and excited. The luminescence is measured as a function of excitation energy and the result is usually compared to a standard phosphor of known quantum efficiency. The losses associated with Stokes shift can be subtracted and the result would be the intrinsic quantum efficiency. Several problems exist with this method of characterization such as backscattered light, coating thickness variability and light trapping. In the case of phosphor powders, the majority of the generated light can escape from the phosphor particles due to their substantially spherical nature and to scattering centers located on or in the material itself. The main problem measuring the efficiency of phosphor powders is backscattering of the light from thick samples. For deposited phosphor films or grown phosphor boules, however, the problem of measuring the phosphor efficiency is affected by light extraction. The majority of the light generated in the phosphor can be trapped within the material itself due to total internal reflection. Several approaches have been used to solve the total internal reflection problem including various roughening techniques and shaping approaches. In these cases, the overall efficiency becomes as much a function of the extraction means as the conversion efficiency. Deposited phosphor films have the added complication of a secondary substrate material with its associated indices and losses.
Mayer et al. in U.S. Pat. No. 6,565,770 describe thin interference pigment flakes that can be made on a flexible substrate and then mechanically removed by flexing the substrate. The dichroic reflectors discussed are used in security enhancement on money and other decorative optical effects. The use of luminescent materials is discussed but is related to the formation of a particular optical effect such as UV illumination for security markings. No explanation for improving the output efficiency of LEDs or other light emitting devices is discussed and no device based on integrating the phosphor layer with the excitation source to form an efficient solid-state lighting element is disclosed.
The use of flake-like phosphors is also discussed by Aoki et al. in U.S. Pat. No. 6,667,574 for use in plasma displays, but the patent again lacks any reference to solid-state lighting applications or methods to enhance their output. In addition, the above two applications are very much cost driven because of the large areas typically required in making a plasma display or the marking of money or decorative items. In order to maximize the performance of these wavelength-converting materials high temperature processing is preferred.
Mueller-Mach et al. in U.S. Pat. No. 6,630,691 disclose a thin single-crystal phosphor substrate onto which an LED structure is fabricated by epitaxial growth techniques. However, single-crystal phosphor substrates are expensive and finding a single crystal phosphor substrate that has the proper lattice match to allow the growth of the LED structure can be difficult.
Ng et al. in US Patent Application No. 20050006659 disclose a planar sheet of a single-crystal phosphor that is placed over the output surface of an LED as a portion of a preformed transparent cap. However, single-crystal phosphor sheets must be grown by epitaxial processes or sliced from bulk single crystals of phosphor material. Single crystal phosphor sheets are therefore too expensive for most practical applications. Planar sheets of polycrystalline phosphors are not disclosed in US Patent Application No. 20050006659. Bonding the planar sheet of a single-crystal phosphor directly to the surface of the LED to improve heat dissipation in the phosphor sheet is also not disclosed.
A need exists to maximize the efficiency of wavelength conversion materials within a solid-state lighting application and to improve the thermal conductivity properties of the materials. In addition, a need exits for low-cost phosphors that have light extraction enhancements and the ability to control the level and type of scatter within the phosphor in order to enhance the overall conversion efficiency.
A need exits for distributed self-cooling light sources which do not require large heavy heatsinks. In recent life costs studies the heatsinks themselves represent a major portion of the life cost. The LED manufacturers have focused on getting more lumens/mm2 to reduce die cost but this increases dramatically the cooling requirements of the heatsink. The authors of this invention have demonstrated LEDs embedded within As such the concept of lumens/gram is disclosed and articles and methods for forming light sources which exhibit high lumens/gram outputs are disclosed.
Present phosphors used in LED applications rely on high dopant concentrations powders within typically an organic matrix to minimize scattering losses. The thermal conductivity is mainly defined by the matrix rather than the phosphor itself with typical thermal conductivity significantly below 1 W/m/K. The industry has also focused on generating more watts/mm2 from the LEDs themselves to decrease cost. There has also been a trend towards larger die size to decrease packaging costs. Unfortunately, this greatly increases the thermal load on the phosphor system. In many cases the phosphor must be remotely mounted such that the watts/mm2 incident on the phosphor and more importantly the matrix is reduced. Inherently the C—H bonds found in virtually all organic systems are susceptible to UV/blue wavelength used to excite the phosphors. In addition, many phosphors act also as photocatalysis further degrading the matrix at the interface between the phosphor powders and organic material. While glass based matrices have been disclosed the efficiency, thermal conductivity, or stability of these systems are lower than polycrystalline, ceramic, or single crystal chips previously disclosed by the authors. In general, powder phosphors within any matrix material does not perform as well as polycrystalline, ceramic, or single crystal luminescent elements.
High dopant concentrations inherently suffer from concentration quenching and as such are inherently less efficient than lower dopant concentrations. In addition, the need exists for methods to distribute the light generated over an area. This is from both an aesthetic standpoint and a safety standpoint. The formation of distributed light sources allows for lower surface brightness and more uniform lighting at a distance. From a safety standpoint, localized point sources represent a greater risk due to the eye's tendency to image these sources on the retina. The need therefore exists for distributed light sources where the surface brightness does not exceed safe and tolerable levels. The authors will disclose how low dopant concentration luminescent elements can be used as lossy waveguides to excitation light such that more uniform distributed light sources are formed.
Based on the above discussion a distributed self-cooling light source is needed. This invention discloses the use of thermal conductivity luminescent elements, articles and methods which meet this need by combining thermal cooling and optical distribution means substantially within the luminescent element itself.